Polyester production from terephthalic acid (TPA) or its esters, such as dimethyl terephthalate (DMT), and glycols is known. This has been accomplished by stagewise melt polymerization of the dihydroxy ester of the bifunctional carboxylic acid, or low molecular weight oligomers, thereof under successively higher vacuum conditions. In order for the polymerization to continue to the degree needed for most commercial applications, the condensation by-products, especially ethylene glycol, must be removed from the reaction system at vacuums as high as 1-3 mm Hg. Such processes require costly high vacuum equipment, multistage steam jets to create the vacuum, and N.sub.2 purged seals and flanges to minimize leakage of air into the system. Condensate from the steam jets and organic by-products from the system end up as a waste water stream that requires treatment and contributes to volatile organic emissions to the air. The present invention provides a less costly polymerization process that can be carried out at atmospheric pressure and in a closed loop configuration that eliminates volatile organic emissions and the waste water discharge.
U.S. Pat. No. 2,973,341 (Hippe) discloses a continuous process for the production of polyester condensate and an improved continuous process for making polyethylene terephthalate from dimethyl terephthalate and ethylene glycol. The process employs liquid dimethyl terephthalate and mixes with it ethylene glycol, in an excess molar ratio of 1.5:1, to form a liquid reaction mixture in a first stage below the transesterification temperature and then carrying the liquid reaction mixture through three separate temperature controlled stages. Transesterification occurs in the second stage at a temperature of not more than 197.degree. C.; vaporous reaction products are removed in the third stage at 197.degree. C. to 230.degree. C. by passing an inert gas through the liquid reaction mixture; polycondensation occurs in the fourth stage at 230.degree. C. to 270.degree. C. for a period of time sufficient to produce a filament forming polyethylene terephthalate condensate while again passing an inert gas through the liquid reaction mixture. Ethylene glycol by-product can be recovered from the fourth stage and recycled to the second stage of the reaction.
U.S. Pat. No. 3,545,520 (Siclari et al.) discloses an apparatus for stripping substances and lightweight fractions from polymers including a means for introducing an inert gas counter current to the polymeric material with the consequent increase in viscosity of the polymers. The apparatus permits recycling a portion of the material removed from the vessel so that the material can be recycled into the reaction container.
U.S. Pat. No. 3,469,618 (Siclari et al.) discloses a method for stripping off volatile fractions from polyamides and polyesters involving feeding material in the form of droplets or liquid threads though an inert gaseous atmosphere, while recirculating that atmosphere.
U.S. Pat. No. 3,110,547 (Emmert) discloses a process for preparing a linear condensation polyester. In one embodiment of the invention, the polymer is extruded downwardly through a chamber while passing a current of inert gas,, such as nitrogen, through the reaction vessel at a rate sufficient to keep the glycol partial pressure below 2 mm Hg while maintaining a temperature between 300.degree. C. and 400.degree. C. in order to rapidly finish the polymer by converting the polymer having a degree of polymerization of from about 15 to 35 to a finished polymer with a degree of polymerization of about 70.
U.S. Pat. No. 3,390,135 (Seiner) discloses a continuous process for preparing polyester wherein the product is contacted with an inert gas which has been passed over the product in a countercurrent manner, at a regulated flow, to remove the water of esterification.